Anaerobic microorganisms in Canada Natural Upgrading Limited (CNUL) fluid fine tailings (FFT) are sustained by residual solvent hydrocarbons. Although FFT are methanogenic in nature, sulfate-reducing microorganisms represent a significant portion of FFT bacterial community. In this study, we examined biodegradation of three iso-alkanes (2-methylbutane, 2-methylpentane, 3-methylpentane), representing major iso-alkanes in paraffinic solvent, in CNUL FFT under sulfate-reducing conditions. During ~1100 d of incubation, only 2-methylpentane was degraded partially whereas 2-methylbutane and 3-methylpentane were not degraded. During active degradation of 2-methylpentane, the bacterial community was dominated by Anaerolineaceae followed by Syntrophaceae, Peptococcaceae, Desulfobacteraceae and Desulfobulbaceae. The archaeal community was codominated by acetoclastic (Methanosaetaceae) and hydrogenotrophic (Methanobacteriaceae) methanogens. This study underlines the limited capability of the microbial community indigenous to CNUL FFT in degrading recalcitrant iso-alkanes under sulfate-reducing conditions.